November 15, 2012

Fossils Of New Ancient Shark Species Reveal Great White Origins

One of the largest living predatory animals and a magnet for media sensationalism, the great white shark has a misunderstood evolutionary history.

Paleontologists have debated that history for the last 150 years, originally classifying the great white as a direct relative of megatooth sharks. A new study from the University of Florida, published in the journal Paleontology, describes and names an ancient intermediate form of the white shark. Carcharodon hubbelli, the intermediate form, shows that the modern white shark likely descended from broad-toothed mako sharks rather than its original classification as a relative of the extinct Carcharocles megalodon, the largest carnivorous shark that ever lived.

The study also concludes the new species was about 2 million years older than previously believed, based on recalibrated dates of the excavation site in Peru.

“We can look at white sharks today a little bit differently ecologically if we know that they come from a mako shark ancestor,” said Dana Ehret, a lecturer at Monmouth University who conducted research for the study as a UF graduate student. “That 2-million-year pushback is pretty significant because in the evolutionary history of white sharks, that puts this species in a more appropriate time category to be ancestral or kind of an intermediate form of white shark.”

The analysis of Hubbell's white shark, C. hubbelli, was based on a complete set of jaws with 222 intact teeth and 45 vertebrae, unlike most ancient sharks which are named using isolated teeth. A collector, Gordon Hubbell, recovered the fossils from a farmer who found them in the Pisco Formation of southern Peru. The species was named after Hubbell who donated the specimens to the Florida Museum of Natural History in 2009.

The impetus of this project was really the fact that Gordon Hubbell donated a majority of his fossil shark collection to the Florida Museum,” Ehret said. “Naming the shark in his honor is a small tip of the hat to all the great things he has done to advance paleontology.”

Ehret and colleagues published an initial study in 2009 in the Journal of Vertebrate Paleontology, describing the species using dates, which reflected information from a 1985 study of the Pisco Formation. Hubbell provided hand-drawn maps and descriptions that allowed the research team to return to the exact site where the fossils were discovered.

The team extracted more accurate age estimates from mollusk shells in the fossil horizon, determining the species was from the late Miocene — about 6.5 million years ago — rather than the early Pliocene — about 4.5 million years ago. These new dates will allow a better understanding of other fossils found in the Pisco Formation, including new whale, marine sloth and terrestrial vertebrate species.

“The thing that was remarkable to me was that these fossils came from right out in the desert and this was before GPS, so Dana had only an approximate notion of where it was,” said Florida Museum of Natural History Director Douglas Jones, who conducted strontium isotope dating of the fossils. “But after a few days of looking, we were able to find this deposit and Dana found the rest of the missing shark´s teeth.”

By comparing the physical shapes of shark teeth to one another, the team was able to determine Hubbell´s white shark was related to ancient broad-toothed mako sharks. Modern white sharks have serrations on their teeth for consuming marine mammals. Mako sharks, on the other hand, do not have serrations because they primarily feed on fish. Coarse serrations on the Hubbell's teeth are indicative of a transition between the mako and the modern white shark.

Researchers have hypothesized these evolutionary relationships for decades, and according to Michael Gottfried, an associate professor in geological sciences at Michigan State University, those who interpret modern white sharks as being closer in relationship to megatooth sharks say it is "a friendly disagreement."

However, “fewer people believe the big megatooth sharks are related to the great white sharks than believe the Earth is flat, "says shark expert David Ward, a research associate at the Natural History Museum, London.

“Everyone working within the field will be absolutely delighted to see this relationship formalized,” Ward said.